AP Biology

Evolution of Populations

Doonesbury - Sunday February 8, 2004

AP Biology

Populations evolve▪ Natural selection acts on individuals

◆ differential survival▪ “survival of the fittest”

◆ differential reproductive success▪ who bears more offspring

▪ Populations evolve◆ genetic makeup of

population changes over time

◆ favorable traits (greater fitness) become more common

Presence of lactate dehydrogenase

Mummichog

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Changes in populationsBent Grass on toxic mine site Pocket Mice in desert lava flows

Pesticidemolecule

Insect cellmembrane

Target site

Resistanttarget site

Insecticide resistance

Target site

Decreased number of target sites

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Individuals DON’T evolve…Individuals survive or don’t survive…Populations evolve

Individuals are selectedIndividuals reproduce or don’t…

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Fitness▪ Survival & Reproductive

success◆ individuals with one

phenotype leave more surviving offspring

Body size & egg laying in water striders

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Variation & natural selection ▪ Variation is the raw material for natural

selection◆ there have to be differences within population◆ some individuals must be more fit than others

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Mean beak depth of parents (mm)

Medium ground finch88 9 10 11

9

10

11

1977 1980 1982 1984

Dry yearDry year

Dry year

Wet year

Bea

k de

pth

Bea

k de

pth

ofof

fspr

ing

(mm

)

Where does Variation come from?▪ Mutation

◆ random changes to DNA▪ errors in mitosis & meiosis▪ environmental damage

▪ Sex ◆ mixing of alleles

▪ recombination of alleles• new arrangements in every offspring

▪ new combinations = new phenotypes◆ spreads variation

▪ offspring inherit traits from parent

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5 Agents of evolutionary changeMutation Gene Flow

Genetic Drift Selection

Non-random mating

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1. Mutation & Variation ▪ Mutation creates variation

◆ new mutations are constantly appearing▪ Mutation changes DNA sequence

◆ changes amino acid sequence?◆ changes protein?

▪ changes structure?▪ changes function?

◆ changes in protein may change phenotype & therefore change fitness

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2. Gene Flow▪ Movement of individuals &

alleles in & out of populations◆ seed & pollen distribution by

wind & insect◆ migration of animals

▪ sub-populations may have different allele frequencies

▪ causes genetic mixing across regions

▪ reduce differences between populations

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Human evolution today▪ Gene flow in human

populations is increasing today◆ transferring alleles

between populations

Are we moving towards a blended world?

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3. Non-random mating▪ Sexual selection

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Warbler

finch

Tree

finc

hes Ground

finches

4. Genetic drift▪ Effect of chance events

◆ founder effect▪ small group splinters off & starts a new colony

◆ bottleneck ▪ some factor (disaster) reduces population to

small number & then population recovers & expands again

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Founder effect▪ When a new population is started

by only a few individuals◆ some rare alleles may be at high

frequency; others may be missing

◆ skew the gene pool of new population▪ human populations that

started from small group of colonists

▪ example: colonization of New World

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Distribution of blood types▪ Distribution of the O type blood allele in native

populations of the world reflects original settlement

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Distribution of blood types▪ Distribution of the B type blood allele in native

populations of the world reflects original migration

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Out of AfricaLikely migration paths of humans out of Africa

Many patterns of human traits reflect this migration

50,000ya

10-20,000ya

10-20,000ya

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Bottleneck effect▪ When large population is drastically

reduced by a disaster◆ famine, natural disaster, loss of habitat…◆ loss of variation by chance event

▪ alleles lost from gene pool• not due to fitness

▪ narrows the gene pool

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Cheetahs ▪ All cheetahs share a small number of alleles

◆ less than 1% diversity◆ as if all cheetahs are

identical twins▪ 2 bottlenecks

◆ 10,000 years ago▪ Ice Age

◆ last 100 years▪ poaching & loss of habitat

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Conservation issues▪ Bottlenecking is an important

concept in conservation biology of endangered species◆ loss of alleles from gene pool◆ reduces variation◆ reduces adaptability

Breeding programs must consciously outcross

Peregrine Falcon

Golden Lion Tamarin

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5. Natural selection▪ Differential survival & reproduction due

to changing environmental conditions ▪ climate change▪ food source availability▪ predators, parasites, diseases▪ toxins

◆ combinations of alleles that provide “fitness” increase in the population▪ adaptive evolutionary change

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5 Agents of evolutionary changeMutation Gene Flow

Genetic Drift Selection

Non-random mating

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Any Questions??